1. Field of the Invention
The present invention relates to a light-receiving circuit which is capable of reducing component counts by simplifying its circuit configurations that read out charges induced by a photodiode, of improving its light-receiving accuracy and increasing a dynamic range of an amount of received light.
The present application claims priority of Japanese Patent Application No. 2005-329477 filed on Nov. 14, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
With recent technological progresses of manufacturing semiconductors, a semiconductor device forming an imaging device made up of a plurality of pixels such as a CMOS (Complementary Metal Oxide Semiconductor) sensor is in use. FIG. 5 is a diagram showing an example of a light-receiving circuit corresponding to a unit pixel in a conventional CMOS sensor. The light-receiving circuit includes a photodiode 100, a first accumulating capacitor 101, a transferring Tr (transistor) 102, a second accumulating capacitor 103, a resetting Tr 104, an amplifying Tr 105, a reading Tr 106, a transferring signal line 107, a resetting signal line 108, a reading signal line 109, and a vertical signal line 110 [see, for example, Patent Reference 1 (Japanese Patent Application No. 2000-092396)].
As shown in FIG. 5, in the conventional light-receiving circuit, the first accumulating capacitor 101 serves as a parasitic capacitor of the photodiode 100 and is connected through the transferring Tr 102 to the second accumulating capacitor 103. The second accumulating capacitor 103 is connected through the resetting Tr 104 to a terminal for a resetting potential ERS and further to a gate of the amplifying Tr 105. A change in a potential in the second accumulating capacitor 103 amplified by the amplifying Tr 105 is read into the vertical signal line 110 connected commonly to pixels in a row direction through the reading Tr 106.
In the light-receiving circuit shown in FIG. 5, charges generated in the photodiode 100 according to input light are accumulated in the first accumulating capacitor 101 and are then transferred, when the transferring Tr 102 is turned ON by a transferring pulse φ TX fed through the transferring signal line 107, to the second accumulating capacitor 103 and are accumulated therein. A change in potential caused by charges accumulated in the second accumulating capacitor 103 is amplified by the amplifying Tr 105 using the resetting potential ERS as a reference potential and is read out, when the reading Tr 106 is turned ON by a reading pulse φ RD, by the vertical signal line 110. A potential of the second accumulating capacitor 103 is reset to be the resetting potential ERS when the resetting Tr 104 is turned ON by the resetting pulse φ RD fed through the resetting signal line 108.
Moreover, a display device is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2004-045879). The disclosed display device includes a pixel array section in which a plurality of signal lines and scanning lines are formed in a ranged manner, a signal line driving circuit to drive signal lines, a scanning line driving circuit to drive scanning lines, a detecting circuit to capture an image, an outputting circuit to output the image, and a sensor controlling circuit to control a sensor to capture images. In the display device, by mounting a plurality of sensors for each pixel to capture images, it is made possible to capture images with high resolution and by accumulating image data captured by the plurality of sensors and to accurately detect an amount of light received by a plurality of photodiodes by storing image data captured by a plurality of sensors into a buffer and further by arranging an array substrate, a facing substrate, and a backlight in this order, it is made to detect, with high accuracy, intensity of light reflected from paper, by using a plurality of photodiodes.
Also, a two-dimensional image sensor is disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. Hei 4-179270), which includes a photosensor array in which pixels each made up of a phototransistor, an accumulating capacitor, and a pixel switching transistor are arranged tow-dimensionally and rows of horizontal switching transistor to sequentially read signals from the photosensor array. The two-dimensional image sensor is so configured that, by mounting each light-shielding film for the pixel switching transistor and the horizontal switching transistor so as to be independent from each of these transistors, each of these transistors is in an electrically floating state.
Furthermore, a thin-film optical sensor is disclosed in Patent Application 4 (Japanese Patent Application Laid-open No. Hei 6-132510) which includes pixels each made up of a thin-film transistor to be used as optical sensor having a plurality of gate electrodes, a thin-film transistor to be for switching, and an accumulating capacitor connected to the thin-film transistor for the optical sensor and the thin-film transistor for switching. The thin film optical sensor is so configured that, by connecting a gate electrode on a drain electrode side of the thin-film transistor for the optical sensor to an electrode of the accumulating capacitor, both the electrodes are maintained at the same potential.
In such a conventional light-receiving circuit as shown in FIG. 5, many circuit elements and wirings including the resetting Tr 104, amplifying Tr 105, resetting signal line 108, wirings for resetting potentials (not shown) are required, thus causing a decrease in a manufacturing yield. Also, the conventional light-receiving circuit shown in FIG. 5 has another problem in that, since an amount of received light is captured as a voltage signal, the captured voltage is affected by a change in temperature, as a result, leading to a narrow dynamic range. Furthermore, a conventional CMOS sensor has still another problem. That is, the conventional CMOS sensor is formed on a silicon wafer via LSI (Large Scale Integration) processing, however, if the sensor is formed on a glass substrate, the sensor capability is affected by variations in characteristics of a TFT (Thin Film Transistor) such as its threshold value. Therefore, the conventional CMOS having LSI circuit configurations cannot be used as the light-receiving circuit to be formed on a glass substrate.